The invention relates to photographic silver halide emulsions. More specifically, the invention relates high bromide, low grain size dispersity tabular grain emulsions precipitated in the presence of a modified gelatin.
In referring to grains and emulsions containing two or more halides, the halides are named in order of ascending concentrations.
The term xe2x80x9chigh bromidexe2x80x9d in referring to grains and emulsions indicates that bromide is present in a concentration of greater than 50 mole percent, based on silver.
The term xe2x80x9cequivalent circular diameterxe2x80x9d or xe2x80x9cECDxe2x80x9d is employed to indicate the diameter of a circle having the same projected area as a silver halide grain.
The term xe2x80x9caspect ratioxe2x80x9d designates the ratio of grain ECD to grain thickness (t).
The term xe2x80x9ctabular grainxe2x80x9d indicates a grain having two parallel crystal faces which are clearly larger than any remaining crystal faces and an aspect ratio of at least 2.
The term xe2x80x9ctabular grain emulsionxe2x80x9d refers to an emulsion in which tabular grains account for greater than 50 percent of total grain projected area.
The term xe2x80x9ccoefficient of variationxe2x80x9d or xe2x80x9cCOVxe2x80x9d is defined as 100 times the standard deviation of grain ECD divided by average grain ECD.
The term xe2x80x9cmonodispersexe2x80x9d in referring to the grain population of a silver halide tabular grain emulsion indicates a COV of less than 25 percent.
The term xe2x80x9csemi-monodispersexe2x80x9d in referring to the grain population of a silver halide tabular grain emulsion indicates a COV of less than 40 percent.
The term xe2x80x9cpHxe2x80x9d is the negative logarithm of the hydrogen ion concentration of a solution.
The term xe2x80x9cpKaxe2x80x9d is the negative logarithm of the thermodynamic acid dissociation constant (Ka) of an acid in solution.
The term xe2x80x9cKaxe2x80x9d is defined by the relationship:
Ka=[[H+][Axe2x88x92]÷[HA] 
where HA represents undissociated acid and H+ and Axe2x88x92 represent dissociated hydrogen ion and anionic moieties, respectively, that together constitute the acid HA.
The term xe2x80x9crobustxe2x80x9d is employed to indicate emulsions that show reduced disparity in grain and performance characteristics from one preparation to the next attributable to inadvertent variances in preparation conditions.
Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
Photographic emulsions contain a dispersing medium and radiation-sensitive grains, which are typically silver halide microcrystals. Although markedly inferior in performance, other silver salts, such as silver thiocyanate, silver phosphate, silver cyanide, silver citrate and silver carbonate, can be precipitated in grain formation, as illustrated by Berriman U.S. Pat. No. 3,367,778, Maskasky U.S. Pat. Nos. 4,435,501, 4,463,087, 4,471,050 and 5,061,617 and Research Disclosure, Vol. 181, May 1979, Item 18153; Ikeda et al U.S. Pat. No. 4,921,784 and Brust et al U.S. Pat. No. 5,395,746.
The radiation-sensitive grains of photographic emulsions are usually formed by reacting a soluble silver salt, such as silver nitrate, with a soluble salt of the halide (or other anion), such as alkali, alkaline earth or ammonium halide. Grain nucleation and growth typically occurs in a dispersing medium comprised of water, dissolved salts and a hydrophilic colloid peptizer such as gelatin and gelatin derivatives. Precipitation can be undertaken under either acid or basic conditions. Under alkaline conditions the ammonium cation can act as a powerful ripening agent, usually resulting in large, highly ripened (sometimes described as spherical) grains. To minimize fog it is usually preferred to maintain a pH either near or on the acid side of neutrality during precipitation. Customarily strong mineral acids, such as nitric, sulfuric or hydrochloric acid are employed; however, other acids have been suggested from time to time for specific applications.
In the early 1980""s it was recognized that a wide-ranging variety of performance advantages can be realized in high bromide silver halide emulsions when at least 50 percent of total grain projected area is accounted for by tabular grains. When interest initially focused on obtaining photographic performance advantages attributable to the tabular grains, the tabular grain emulsions contained a high proportion of nontabular grains, and the emulsions exhibited a high degree of grain size dispersity, attributable to the mixture of grain shapes as well as differences in the sizes of the tabular grains.
About a decade after the initial recognition of wide-ranging performance advantages for high bromide tabular grain emulsions, it was discovered that the presence of polyalkylene oxide block copolymer surfactants present during the formation of grain nuclei consisting essentially of silver bromide can significantly increase the proportion of the total grain population accounted for by tabular grains (e.g., where tabular grains account for greater than 90 percent of total grain projected area) and produce relatively monodisperse emulsions. These modified precipitation techniques allowed COV""s of less than 40 percent, based on the total grain population, to be realized consistently. In fact, monodisperse emulsions with COV""s of less than 25 percent based on total grains, and even extraordinary levels of monodispersity with COV""s based on total grains ranging below 10 percent, were realized. Further, in these emulsion precipitations, tabular grains usually account for xe2x80x9csubstantially allxe2x80x9d (defined as  greater than 97%) of total grain projected area. Preparations of relatively monodisperse high bromide tabular grain emulsions employing polyalkylene oxide block copolymer surfactants are illustrated by the following: Tsaur et al U.S. Pat. No. 5,147,771; Tsaur et al U.S. Pat. No. 5,147,772; Tsaur et al U.S. Pat. No. 5,147,773; Tsaur et al U.S. Pat. No. 5,171,659; Tsaur et al U.S. Pat. No. 5,210,013; Tsaur et al U.S. Pat. No. 5,252,453; Kim et al U.S. Pat. No. 5,272,048; and Fenton et al U.S. Pat. No. 5,476,760.
Although polyalkylene oxide block copolymer surfactants consistently increase the percentage of projected are accounted for by tabular grains and reduce the grain dispersity of high bromide tabular grain emulsions, Brust et al. U.S. Pat. No. 5,763,151 discloses that these surfactants are susceptible to allowing batch to batch variations in tabular grain mean thicknesses and ECD""s when emulsion precipitation conditions are inadvertently varied during emulsion manufacture. In other words, the preparation processes have shown themselves to lack the degree of robustness desired using customary manufacturing control practices. Brust et al. discloses a method for improving the robustness of such preparation processes wherein the silver halide grain nuclei are grown at a pH in the range of from 3.0 to 8.0 and in the presence of at least a 0.01 M concentration of a partially dissociated acid having a pKa that is within 2.5 units of the growth pH and that forms a silver salt more soluble than the silver halide incorporated in the grains.
It would be desirable to provide alternative methods for further improving the robustness of high bromide silver halide tabular grain emulsions grown in the presence of polyalkylene oxide block copolymer surfactants.
In one aspect this invention is directed to a radiation-sensitive emulsion comprised of an aqueous dispersing medium and a coprecipitated grain population including tabular grains containing greater than 50 mole percent bromide, based on silver, having {111} major faces, and accounting for greater than 90 percent of total grain projected area, wherein said dispersing medium is comprised of (a) a gelatin which has been modified to convert at least one carboxylic acid group thereof to a group that does not exhibit pH-dependent ionization within the pH range from 4.0 to 7.0, and (b) a polyalkylene oxide block copolymer surfactant.
In a further aspect this invention is directed to a process of preparing a photographic emulsion having silver halide grains including tabular grains containing greater than 50 mole percent bromide, based on silver, having {111} major faces, and accounting for greater than 90 percent of total grain projected area, said process comprising: forming in the presence of a dispersing medium containing gelatin and a polyalkylene oxide block copolymer surfactant a population of silver halide grain nuclei containing twin planes, and growing the silver halide grain nuclei containing twin planes in the dispersing medium to form tabular silver halide grains, wherein (a) gelatin in the dispersing medium comprises a modified gelatin of the formula Gel-C(O)-G where Gel represents a gelatin polypeptide, -C(O)- is a carbonyl group from a free carboxyl moiety of an aspartic acid or a glutamic acid component in the polypeptide, and G is a substituent which is free from groups having a pKa of from 3 to 8, and (b) the silver halide grain nuclei are grown at a pH in the range of from 3.0 to 8.0.
A primary feature of the invention is the recognition of the impact of the pH sensitivity of free carboxy groups of gelatin polypeptides upon the resulting robustness of the high bromide tabular grain preparation process.
Further aspects of the invention can be appreciated by reference to the following detailed description, including the Examples containing emulsions prepared in accordance with the invention.